1. Field of the Invention
The present invention relates to a projection exposure apparatus used in the manufacture of semiconductor elements and liquid-crystal displays and, in particular, a projection exposure apparatus which includes an off-axis type plate alignment system.
2. Description of Related Art
In a projection exposure apparatus having an off-axis type plate alignment system, a fiducial mark member having a reference mark is fixed on a plate stage which moves two-dimensionally while holding a photosensitive plate consisting of a wafer, glass plate, or similar element coated with a sensitizer such as a photoresist. The fiducial mark members are used to control a baseline value of the distance between the off-axis alignment system and the projection optical system.
FIG. 5 is a diagram which illustrates the principles of baseline measurement with a conventional projection exposure apparatus. As illustrated in FIG. 5 (a), reticle marks (RMa) and (RMb) are formed on a mask (10) at positions which are symmetrical with respect to the mask center (C). The mask (10) is held by a mask stage (11). The mask stage (11) is moved so as to align the mask center (C) of the mask (10) with the optical axis (AX) of a projection optical system (20). A fiducial mark member (FP), comprising a fiducial mark (FM) which is equivalent to an alignment mark formed on the surface of the photosensitive plate, is attached on a plate stage (31) at a position in which it does not interfere with the photosensitive plate. When the plate stage (31) is positioned so that the fiducial mark (FM) moves to the desired position in the imaging field of the projection optical system (20), the mark (RMa) of the mask (10) and the fiducial mark (FM) will be detected at the same time by a TTL (through-the-lens) type mask alignment system (50a) situated above the mask (10). In addition, when the plate stage (31) is moved to another position, it is possible to detect the mark (RMb) of the mask (10) and the fiducial mark (FM) at the same time by a mask alignment system (50b).
An off-axis type plate alignment system (60) is rigidly mounted outside of the projection optical system (20) (outside the imaging field). The optical axis of the plate alignment system (60) is parallel to the optical axis (AX) of the projection optical system (20) on the projected image plane. A target mark, serving as a reference in aligning the marks on the photosensitive plate or the fiducial mark (FM), is formed on a glass plate inside the plate alignment system (60), and is arranged substantially in conjugation with the projected image plane (the photosensitive plate surface or the surface of the fiducial mark (FM)).
As illustrated in FIG. 5 (b), a laser interferometer is used to measure the position X.sub.1 of the plate stage (31) when the mark (RMa) of the mask (10) and the fiducial mark (FM) on the fiducial mark member (FP) are aligned by the mask alignment system (50a). Likewise, a laser interferometer and other elements are used to measure the position X.sub.2 of the plate stage (31) when the mark (RMb) of the mask (10) and the fiducial mark (FM) are aligned by the mask alignment system (50b) as well as the position X.sub.4 of the plate stage (31) when the target mark of the plate alignment system (60) and the fiducial mark (FM) are aligned. If the midpoint between the positions X.sub.1 and X.sub.2 is X.sub.3, then the position X.sub.3 is located on the optical axis (AX) of the projection optical system (20), and is in a position conjugate with the reticle center (C).
The baseline value (BL) is determined by calculating the difference (X.sub.3 -X.sub.4). The baseline value (BL) is the reference value for subsequently aligning the alignment marks on the photosensitive plate using the plate alignment system (60) and feeding them immediately beneath the projection optical system (20). More specifically, if the distance between the center of the single shot (exposed area) on the photosensitive plate and the alignment mark on the photosensitive plate is XP, and if X.sub.5 is the position of the plate stage (31) when the alignment mark on the photosensitive plate is matched with the target mark of the off-axis type plate alignment system (60), then the plate stage (31) should be moved to the position obtained from the following expression in order to match the shot center with the mask center (C). EQU (X.sub.5 -BL-XP) or (X.sub.5 -BL+XP)
Thus, after the alignment mark position on the photosensitive plate is detected using the off-axis type plate alignment system (60), it is only necessary to feed the plate stage (31) by a certain amount related to the baseline value (BL) in order to immediately and accurately position the pattern of the mask (10) over the shot area on the photosensitive plate for exposure. It should be noted that although this discussion pertains only to one dimension, in actual practice it is necessary to consider two dimensions.
Conventional projection exposure apparatuses situate only one fiducial mark member with a fiducial mark in a position which does not interfere with a photosensitive plate on a plate stage. As a result, various problems arise. As the size of the photosensitive plate increases, the plate stage begins to have an unnecessarily long stroke in order to perform exposure. The size of the photosensitive plate is otherwise restricted by a limited stroke. Restrictions on the positioning of the alignment system, which handles positioning between layers that are stacked and exposed, are also present.
Projection exposure apparatuses used in the manufacture of, for example, liquid-crystal displays produce increases in display sizes and have caused an increase in the size of the photosensitive plate (glass plate). As a result, the size of the plate stage on which the plate is set has also increased, gradually lengthening the stroke of the plate stage. At the same time, the alignment marks formed on the plate are, in many cases, situated on the periphery of the plate for purposes such as increasing the span between marks for improved measurement accuracy, or integrating the alignment process through the formation of marks which are always in the same position on the plate regardless of the device layout. When a single off-axis plate alignment system is used to detect alignment marks situated on both ends of this type of large plate, the stroke of the plate stage must be made extremely long. This makes plate stage design difficult. At the same time, it increases the likelihood of a decrease in throughput. The use of off-axis plate alignment systems at both ends of the projection optical system may be considered in order to avoid increasing the length of the plate stage stroke. Although the use of two plate alignment systems does not require a long stroke during the alignment process, however, the plate stage must still have a long stroke in order to calibrate the two plate alignment systems using fiducial marks through conventional techniques.